Digital data transmission provides high-speed communications for a variety of applications. Digital communication over existing telephone system infrastructure, e.g., twisted pair lines, has been implemented to provide service to homes and businesses accessible over existing telephone lines. Various digital communication systems, such as digital subscriber line (DSL), have associated standards enabling communication between users over twisted pair lines.
DSL modems use frequencies above the traditional voice band to carry high-speed data. Discrete multitone (DMT) modulation is a technique used to partition a DSL communication channel into N independent additive white Gaussian noise (AWGN) subchannels. Dividing the DSL channel into subchannels reduces intersymbol interference (ISI), thus reducing complexity at the receiver. However, partitioning the channel into multiple carriers introduces the problem of determining the power and data rate of each carrier at the transmitter.
Existing telephone lines are typically arranged in a binder with a number of wire pairs in each binder. Crosstalk interference between twisted pairs arising from electromagnetic coupling within the binder may degrade the communication signals. When the level of crosstalk is high, the power allocation of a user changes the noise experienced by the other users in the same binder. Near end crosstalk (NEXT) refers to crosstalk created by transmitters located on the same side as the receiver. Far end crosstalk (FEXT) refers to crosstalk created by transmitters on opposite sides. NEXT is typically much larger than FEXT. Crosstalk can be the limiting factor in determining the data rates of offered DSL services at various loop lengths. In this case, the performance of DSL modems can be improved by jointly considering the bit and power allocation of all users.
DSL channels are significantly frequency selective, and effective DSL power allocation schemes take into account the allocation of power in each carrier frequency as well as the total amount of power allocated for each user. Some types of DSL, e.g., very high speed DSL (VDSL) are particularly affected by a near-far problem when two transmitters located at different distances from the central office both attempt to communicate with the central office. When one transmitter is much closer to the central office, the interference due to the closer transmitter may overwhelm the signal from the far transmitter. As the distance from the central office increases, the signal quality decreases and the connection speed goes down.
Various techniques have been devised for allocating power and rate to subchannels to improve performance in multi-carrier systems. These techniques involve, for example, rate maximization to increase the data rate under a fixed data constraint, or margin maximization, wherein the transmit power is minimized for a fixed rate. Many of these techniques are suboptimal and computationally efficient, or are optimal, but computationally prohibitive. There is a need for practically implementable rate and power allocation solutions that address the above problems.